Ground slips 1 metre as quake pummels Christchurch

Based on the magnitude, Gary Gibson, a seismologist at the University of Melbourne, Australia, says ground shaking would have been “severe” in the city centre, which was only 10 kilometres from the earthquake’s epicentre.

Details of the damage remain vague, but 65 deaths have been confirmed and there are widespread reports of severe building damage. Two buses were crushed by collapsing buildings, and 75 per cent of the city was left without power.

According to Gibson, the active fault area was approximately 225 square kilometres. “One side of the fault moved about 1 metre relative to the other,” he says.

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When weaker is worse

Last year’s 7.1-magnitude earthquake was more than 10 times as strong as today’s but caused no deaths, probably because it occurred at greater depth and further away from Christchurch&colon; its epicentre was 40 kilometres west of the city. And the focus of September’s quake was some 10 kilometres below ground – today’s was half as deep.

“The ground motion [of the previous quake] had significantly attenuated by the time it reached Christchurch,” says Adam Pascale, a seismologist at Environmental Systems & Services in Richmond, Australia.

More to come?

The question now is&colon; are more quakes likely? That all depends on whether the underlying rocks have settled into a position where they can absorb the stress from tectonic plate movements again. Gibson suspects this has already happened. “An earthquake of this magnitude does a good job of releasing that stress,” he says. Although it is possible that other fault lines in the region might burst, “it’s unlikely”.

New Zealand sits on the tectonic boundary between the Pacific plate and the Australia-India plate. Christchurch is not on that frontier, but it is near to related secondary faults such as the Alpine fault, which runs along the spine of the South Island.

When this article was first posted, we incorrectly quoted Gary Gibson’s assessment of the rupture area. We also incorrectly stated the earthquake’s epicentre was 70 kilometres west of Christchurch. It was the centroid (the mid-point of energy release from the rupture) that was 70 kilometres west of the city – the epicentre (the point on the Earth’s surface vertically above the point where rupture began) was just 40 kilometres west.